Radio-wave detector for discovering the movement of persons or objects in a confined space

ABSTRACT

Movements of persons or articles in a monitored space are detected by a change in the effective capacitance between an antenna and an associated counterpoise defining that space, the antenna being energized by an oscillator whose tank circuit is tuned to a predetermined radio frequency f&#39;&#39;&#39;&#39;e. The oscillator works into a tuned monitoring circuit resonant at a different frequency fo, a capacitive feedback path extending from a tap on the inductive branch of that circuit to an input of the oscillator for applying thereto a control voltage which shifts its operating frequency from f&#39;&#39;&#39;&#39;e to a value fe closer to fo. This shift in oscillator voltage is reduced by a lowering of the control voltage through a further detuning of the monitoring circuit by a movement to be detected, with resulting change of the operating frequency to a value f&#39;&#39;e between f&#39;&#39;&#39;&#39;e and fe whereby the change in output voltage due to such detuning is intensified. A load circuit connected to the monitoring circuit includes a normally de-energized relay whose energization in response to the aforementioned voltage change produces a voltage drop across a supply resistor common to the relay and the oscillator whereby this voltage change is further stepped up.

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Salmet RADIO-WAVE DETECTOR FOR DISCOVERING THE MOVEMENT OF PERSONS OROBJECTS IN A CONFINED SPACE [76] Inventor: Gaston Raoul Salmet, 15avenue des Arlantes, Saint-Maur, Val de Marne, France [22] Filed: Mar.15, 1973 [21] Appl. No.: 341,744

[30] Foreign Application Priority Data [111 3,828,335 [451 Aug. 6, 1974,Zt; i)

jrimary Examiner-David L. Trafton [5 7] ABSTRACT Movements of persons orarticles in a monitored space are detected by a change in the effectivecapacitance between an antenna and an associated counterpoise definingthat space, the antenna being energized by an oscillator whose tankcircuit is tuned to a predeter mined radio frequency f 'e. Theoscillator works into a tuned monitoring circuit resonant at a differentfrequency f,,, a capacitive feedback path extending from a tap on theinductive branch of that circuit to an input of the oscillator forapplying thereto a control voltage which shifts its operating frequencyfrom f"e to a value fe closer to f,, This shift in oscillator voltage isreduced by a lowering of the control voltage through a further detuningof the monitoring circuit by a movement to be detected, with resultingchange of the operating frequency to a value fe between 1 'e and fewhereby the change in output voltage due to such detuning isintensified. A load circuit connected to the monitoring circuit includesa normally deenergized relay whose energization in response to theaforementioned voltage change produces a voltage drop across a supplyresistor common to the relay and the oscillator whereby this voltagechange is further stepped up.

11 Claims, 8 Drawing Figures RADIO-WAVE DETECTOR FOR DISCOVERING THEMOVEMENT OF PERSONS OR OBJECTS IN A CONFINED SPACE This inventionrelates to a radio-wave detector for discovering the movement of peoplesor articles in a confined space.

It has already been proposed that the movement of people or objects in aconfined space be detected with the aid of radio waves in response tothe alteration produced in the field reaching a receiver from atransmitter when a person or article enters or leaves the space in whichthe radio transmission between the transmitter and the receiver takesplace. A system of this kind is of course fairly expensive, needing asit does two complete radio frequency transducers, namely a transmitterand a receiver.

The object of my invention is to provide a detector of this nature usingjust a single such transducer, i.e., a transmitter. In accordance withmy present invention, the space to be monitored is defined by an antennaand an associated counterpoise connected across a tuned circuit which iscoupled to an oscillator whose tank circuit is tuned to a predeterminedradio frequency, designated f 'e hereinafter, differing from theresonance frequency of that tuned circuit. Owing to the connection ofthe latter circuit to the antenna and its counterpoise, resonancefrequency f is codetermined by the effective antenna capacitance whichin turn is variable by a movement to be detected in the monitored space,the sense of capacitance variation due to such move-' ment being sochosen as to increase the difference between the two frequencies f 'eand f,,. A feedback path between the tuned monitoring circuit, servingto energize the antenna, and the oscillator delivers to an input of thelatter, such as a transistor base, a control voltage establishing anoperating frequency fe intermediate and frequencies fl, and f 'e, thiscontrol voltage varying with changes in the effective antennacapacitance for shifting the operating frequency fe toward frequencyf'e, i.e., to a new value fe more remote from resonance frequency 11,,in response to a movement to be detected; the resulting voltage changein the tuned monitoring circuit actuates a responder in a load circuit,connected to the monitoring circuit, for indicating that change.

Advantageously, pursuant to a further feature of my invention, the loadcircuit includes a differentiation network of long time constant (e.g.seconds); a diode in this network may serve to bypass voltage changes inthe monitoring circuit whose polarity is opposite that caused by amovement to be detected.

In a system of this kind, when the effective capacitance between theantenna and its counterpoise is constant or slowly varying, no signalreaches the responder which preferably includes a relay triggerable byan amplifier; when, however, that capacitance varies at a critical rate,a signal is applied to that amplifier to energize the relay.

Preferably, the amplifier comprises an integrator so that the devicedoes not respond to interference picked up via the antenna. Theintegrator must have a time constant longer than the average very briefduration of domestic, industrial and atmospheric interference.

In cases in which the device is to detect the presence of living beings,more particularly people, whose approach acts to increase the circuitcapacitance and also to damp the tuning circuit, the device is soadjusted that the transmitter operating point is positioned on thedescending part of the resonance curve of the tuning circuit i.e., at afrequency a little beyond the exact resonance frequency. The reason forthis is that since the capacitance increase in the tuned circuit is theresult of the presence of a living being in the transmitter field, sothat the resonance frequency of this circuit decreases, and since aliving body has a small dielectric constant and therefore increasesdamping, the difference in transmitted energy due to the displacement ofthe resonance curve toward the origin and to the flattening thereof isat a maximum. This appreciable reduction in the transmitted field isdetected by the differentiation circuit which actuates the responder.

The deactivation of the responder can be delayed, preferably via thedifferentiation network, by using negative detection and by energizingthe complete system via a common resistance such as the internalresistance of the power supply or an auxiliary resistance in seriestherewith.

When, as a result of detecting a movement, an integrating network inseries with the differentiation circuit registers a voltage drop(negative detection) and triggers the responder, the resulting powerconsumption causes an even greater voltage drop at the output of theintegrating network; of the detector, owing to this large voltage dropand the long time constant of the differentiator cascaded with theintegrator, upon termination of the triggering event (movement of aperson) the differentiating capacitor will take much longer than thattime. constant to return to its former state of charge and thus stop theoperation of the responder. Once the system has ceased to operate, itcan be restored to its normal supervisory or monitoring state withremoval of the charge associated with normalization of the voltage.

The system according to the invention is of use for protecting themonitored premises or other spaces from intruders and for transmittingrecorded messages for advertising, tourist and museum purposes. In thelatter case delayed operation of the responder can be produced andmaintained by detection, with a short time constant, of message signalsgoing to a loudspeaker, so that the delay ceases when the speech orsound transmission ceases and the device returns to its supervisory orstand-by state.

My invention will now be described in detail with reference to theaccompanying drawing in which:

FIG. 1 is a diagrammatic view of a systemaccording to the invention;

FIGS. 2a and 2b are diagrams explaining the operation of the system ofFIG. 1;

FIG. 3 is an equivalent circuit diagram of the transmission circuit ofthe system;

FIG. 4 is a partial block diagram of the complete system;

FIG. 5 is a diagram of its power supply;

FIG. 6 is a diagram showing how deactivation can be delayed when thesystem is used with a loudspeaker;

FIG. 7 is a simplified detail of the diagram shown in FIG. 4.

In FIG. 1 I have shown a resonant monitoring circuit comprising aninductance L and a capacitor C. One side of the circuit is connected toan antenna A and the other to a counterpoise therefor (here ground). Theactual capacitance of the tuned circuit is therefore not C but C Ca, Ca,denoting the antenna-to-counterpoise capacitance. The resonant frequencyis therefore:

f =1/(21r V L (C +Ca)) Let us assume now that a person P approaches theantenna; since the human body is a conductor, it will increase theantenna-to-counterpoise capacitance so that the resonant frequency ofthe tuned circuit be comes:

f= l/(2rr V L (C +Ca +Cx)) Cx denotes the extra capacitance due to thepresence of person P.

If the circuit L-C is energized by a constantamplitude butvariable-frequency alternating current I, the AC voltages at the circuitterminals in dependence upon frequency are represented by a curve K inFIGS. 2a and 2b. Ifthe capacitance increases, the curve which representsthe frequency F is the dotted-line curve K, of FIG. 2a. If, in thiscase, the transmission frequency f is set at a value fe slightly greaterthan the resonant frequency f,,, the capacitance increase and thereforethe frequency variation will of course produce a variation AV of the ACvoltage across the circuit. Owing to the shape of the resonance curve,maximum sensitivity is obtained for a value of the frequency fe suchthat the normal operating voltage V, developed in circuit L-C isapproximately 80 percent of the resonant voltage V Also, for a givenchange in capacitance the voltage variation AV, is proportional to the Qfactor of the circuit and inversely proportional to the tuningcapacitance C. A very-high-Q inductance and a low capacitance shouldtherefore be used. To give some idea about system sensitivity, if C 500Pf and Q 200, a 0.02Pf variation of C gives a relative variation AV,/V,,of approximately 1 percent.

Since the human body has a high ohmic resistance, this choice ofposition for the frequency fe is advantageous in the case of a humanbeing entering the field.

As the equivalent circuit diagram of FIG. 3 shows, the extra capacitanceCx corresponding to the presence of person P and applied across thecircuit is in series with a considerable series resistive component Rx,which helps to damp the circuit and therefore to change the curve K,into the dotted-line curve K i.e., to reduce the voltage across thecircuit. This leads to an extra voltage reduction AV so that the totalvariation due to a person entering the field becomes:

AV AV, AV,

The damping efiect is therefore cumulative with the effect of the extracapacitance if the frequency fe is above the frequency f,,.

In some cases, however, the movement to be detected causes a decrease incapacitance, as for instance in the case of a surreptitious opening ofan armored door, metal shutter, grid or closure lattice; in this case,as shown in FIG. 2b, the oscillator frequency fe is advantageouslysmaller than the resonant frequency f, of the transmitting circuit L-C.When the capacitance across the inductance L decreases, the resonancecurve shifts to the right (curve K and as in the previous case thereoccurs a voltage reduction AV,; in this instance, however, there is nodamping variation.

In both cases (FIGS. 20 and 2b) the value of AV, can be increased to AVas will be shown hereinafter. Conversely, to reduce the systemsensitivity to prevent accidental operation, the offset between thefrequencies fe and f can be increased so that operation is shifted tothe skirts of the curve K. The choice of the offset or differencebetween the frequencies fe and f, determines therefore the sensitivityof the system.

These considerations are used in the design of a detector Det for whicha circuit diagram is shown in FIG. 4. A fixed-frequency oscillator 0works through an adjustable resistance R, into an amplifier A, which hasa high internal output impedance so as not to damp the oscillatoryoutput circuit consisting of an autotransformer L, preferably of theferrite-core kind, and a capacitor C,. The lower terminal end of theoscillatory circuit is connected to the counterpoise i.e., to ground inthe present case whereas its upper terminal is tied to the antenna A.The resistance R, serves to control the drive of the amplifier A, sothat at resonance of the circuit L-C, the AC voltage between a point ain the amplifier output and ground is near the maximum which theamplifier A, can provide without being saturated.

The amplifier A, therefore behaves like a constantcurrent generator. Thetap a is so chosen that the total supply direct current under theseconditions does not exceed a given low value, e.g. 1 mA, when theapparatus is on standby, so that the system can run for a long time(several months) if battery-energized.

FIG. 7 is a very simplified circuit diagram of the oscillator O and theamplifier A. The oscillator O is a Hartley circuit comprising atransistor Tr,, a ferrite-core inductance L, and a capacitor C theelements L, and C forming a tuned circuit. Depending upon the number ofstages in it, the amplifier A either inverts or does not invert thephase of the signal which it transmits; in this particular case theamplifier A, which comprises a single transistor Tr inverts the phase. Afeedback coupling between the tap a and a point g (i.e., the base leadof the transistor Tr, of the oscillator O) is provided by a capacitor C,which therefore feeds back a control voltage that is always in phasequadrature with the normal voltage on lead g.

Ishall first describe the case of FIG. 2a and of an amplifier whichinverts the phase of the output voltage relatively to the input voltage.In the absence of feedback coupling the oscillator frequency fe would bethe same as the frequency of the tuned circuit L,-C constituting thetank circuit of oscillator 0.

If the circuit L-C, is tuned exactly to the oscillator frequency fe (ffe), the -out-of-phase control voltage fed back via capacitor C to pointg lags with reference to the regenerative feedback voltage from tankcircuit L,C, present at point g in the absence of such capacitivefeedback, and so the oscillator frequency is altered. It can be shownthat the operating frequency decreases in this case. Conversely, if theoscillatory detector circuit L-C,, is detuned, the lagging controlvoltage decreases and the frequency fe increases, tending toward thenatural frequency f'e of the tuned circuit L,-C,.

Consequently, the movement of curve K towardK; when capacitance C22 is66666151 in parallel with capacitances C and Ca (FIG. 3) is enhanced bythe effect of frequency fe shifting to a higher value f'e (FIG. 2a),closer to the natural frequency f 'e of tank circuit L C and so thedetectable voltage variation becomes AV instead of AV,.

Thanks to the high Q of the circuit LC,, which gives a very sharplypeaked curve K, the variation AV and therefore the sensitivity of thesystem can be increased by a factor of approximately 3 to 5.

In the case of FIG. 2b (detuning of circuit L-C by decrease ofcapacitance), the circuit arrangement shown in FIG. 7 again increasesthe frequency fe, but in this case such increase counteracts the effectof curve K shifting toward K To obtain a similar effect i.e., avariation of oscillator frequency fe in the sense opposite to thevariation of the detector-circuit frequency the feed-back circuit shouldbe connected to a point g which is symmetrical with reference to thepoint g relatively to the neutral point h of the oscillator tunedcircuit. Of course, the connections to points g and g must be reversedif the amplifier does not invert the phase of the output voltage.

Because of the autotransformer effect of the inductance L of thedetector circuit, the voltage between the point a and ground is normallyseveral tens of volts. A proportion of this voltage is taken off at atop b (FIG. 4) and fed via a rectifying connection, constituted by adiode D to an integrating circuit comprising a resistance R in parallelwith a capacitor C As will be apparent, the alternating voltage of radiofrequency fe (or f'e) developed in the tuned circuit L-C, builds up anegative potential, of a magnitude proportional to the radio-frequencyvoltage, on the ungrounded terminal of integrating capacitor CAutotransforrner tap b is so chosen that the voltage thus detected islarge but its detection does not cause appreciable damping of thecircuit LC As an example, the DC voltage across the resistance R can besomething like 50V, whereas the AC voltage between the tap a and groundmay be only about 5V r.m.s. Of course, and as explained with referenceto FIG. 2a, in most cases the inductance L is adjusted to above theresonance frequency (or if such adjustment cannot be provided, thecapacitance C is so adjusted) to give a voltage across the circuit L-C,of about 80 percent of the voltage at resonance.

The frequency chosen is approximately 30 KHz, which is low enough forthe transmitted voltage and its harmonics not to interfere with radiobroadcasting, yet high enough to be able to use high-Q inductances ofreduced size.

The negative voltage detected at the ungrounded terminal c of network RC is transmitted to a DC amplitier A via a differentiation network (I -RThe function of this network, whose time constant is on the order ofseconds, is to transmit at a point d only relatively rapid variations ofthe voltage at point c signaling the approach of a person, and not totransmit very slow variations, due for example to variations of theambient temperature or of the supply voltage (upon exhaustion of thecells).

Since the voltage at c is negative and the approach of a person reducesthis voltage, such approach produces a positive voltage at d.

The amplifier A energizing a relay R is so designed that for zero ornegative voltage at d the voltage across the relay R1 is zero, whereasfor even a very small positive voltage (e.g. on the order of 0.1V) atthe point d the amplifier A energizes the relay Rll sufficiently for thesame to become operative.

Preferably, the input impedance of the amplifier A is very high severalmegohms so that a very high detected voltage can be produced at thepoint c with low power consumption. The magnitude of the resistance Rshould therefore itself also be very high.

Advantageously, the amplifier A is a conventional NPN transistorconnected as a cathode follower, so as to have a high input impedance,followed by another NPN transistor arranged as a voltage amplifier, inturn followed by a voltage-amplifying PNP transistor driving the relay ROne of the stages of amplifier A includes an integrating network Int,having a time constant of 0.2 to 0.5 sec, for general interferencesuppression.

Relay R1 has two contacts r r the former actuating a responder, e.g.triggering an alarm AL, whereas the latter preserves the response by theapplication of an appropriate voltage +v to one of the transistors ofamplifier A This obviates the need for a direct holding contact on therelay; the holding effect of voltage +v can be controlled by anyparameter, e.g. as described below with reference to FIG. 6.

Since the circuit arrangement responds to the appearance of a positivevoltage at the point d, a diode D can provide very rapid absorption ofnegative potential variations at that point so that when such variationsoccur, for instance, at switch-on, the apparatus is immediately readyfor operation i.e., there is zero voltage at the point d.

Detection of increasing rather than decreasing voltage swings at theinput terminal b is possible if the amplifier A is sensitive to negativevoltages (PNP stage), in which case the shunt diode D would be inverted.

If the detector is self-restoring to the standby or monitoring state, itis preferable for many uses of the invention to have a signal of limitedduration rather than a steady signal. Various auxiliary means are knownfor providing a delay giving a signal lasting for a few tens of seconds;in the present case, however, there is a very simple way of achievingthis result with virtually no addition of extra items.

The delay procedure can be clearly understood from the followingexample.

If the system consumes, say, 1 mA on standby, its consumption is e.g. 30mA when relay R1 operates, because of the energy used up by this relay.If a resistance Rs (FIG. 5) is connected in series with the associatedpower supply S and is of such magnitude that the supply voltageenergizing the detector part Det of the system drops by e.g. 10 percentwhen the relay is thus energized, all the AC and DC voltages willdecrease in substantially the same proportion. More particularly, thevoltage integrated at the point c, which was 50 V, becomes -45 V, andthe initial voltage change which triggered the alarm and which was justa few tenths of a volt is converted into a much greater swing as aresult of the voltage drop developed across resistor Rs. A positivevoltage above 5 V therefore appears at the point d. To dissipate thisvoltage by way of the differentiation network, capacitor C must firstdischarge through resistor R sufficiently for the voltage at d todecrease to e.g. 0.1 V or less. When this voltage has been reached,

the relay R1 returns to normal and stops the responder.

Consequently, if the value of capacitance C is chosen appropriately, asignal lasting for approximately 1 minute can be produced without anyother delay means being used.

Conversely, however, when the relay R1 releases, a very large negativevoltage variation occurs at the point d; as previously described, thisvariation is, however, absorbed very rapidly by the diode D and thedetector is ready almost immediately for further operation.

Since the detector operates basically on the principle of varying thestate of tuning of a tuned transmitting circuit, the main variationbeing capacitive and the secondary variation being in the damping, thereis no need to use a vertical antenna in association with a counterpoiseforming a horizontal mass plane. The antenna its counterpoise can bee.g. two metal strips or even wires connected to the two ends of theoscillatory circuit and extending parallel to each other. The strips canbe placed on the ground, if the same is not conductive (floor), orpositioned vertically on either side of an entrance which it is requiredto protect. The sensing element formed by the antenna and itscounterpoise can be devised differently to suit individual cases. Interalia, in a room or the like the antenna can be in the ceiling and thecounterpoise can be below it on the floor.

Thanks to its high sensitivity, the system is highly versatile. Forinstance, with a vertical antenna 1.50 meters long and a ground planewhich is either inherently conductive or made so, e.g.- by latticework,a person can be detected at up to about 8 meters from the antenna i.e.,assuming that the antenna is accessible from all directions, theoperative area of the system is on the order of 200 m.

An obvious use of the system is for protection against unwantedintrusions. It can also be used to detect movement, e.g. for automaticdoor opening, lighting of passageways (timers), or counting people.

Also, its delay feature makes it very suitable for advertising purposesas, for instance, to trigger a tape recorder which broadcasts anadvertising announcement or a commentary on an article on show in amuseum.

FIG. 6 shows one such adaptation of the invention. An endless magnetictape m contains a text which may be repeated a number of times, thespacing between repetitions being such that the end of the text and thestart of its repetition are separated by an interval of a few seconds.For instance, in its commercial application a tape recorder MAGcontaining the tape m is under the control of the movement detectorhereinbefore described. The two systems are located near the place wherepossible clients may pass by. When a client approaches, the relay of thedetector Det starts the tape recorder MAG. For a brief period the taperecorder transmits no signal to loudspeaker H, but the relay R1 remainsheld by the delay means hereinbefore described; in this case the delayis fairly short, e.g. seconds. At a predetermined time the tape recorderMAG starts to read out the text through a loudspeaker H. After some timethe internal delay of the detector Det terminates, but the detectorrelay R1 locks as a result of detection of the transmitted modulation;the voltage across the loudspeaker H is detected by a network DAL-C, andapplied through holding contact r, of relay R1 of FIG. 4 to hold therelay while the lowfrequency modulation i.e., the transmitted messagecontinues. Upon cessation of the message the holding voltage disappears,the relay R1 releases and the tape recorder MAG stops. The procedure canrestart when someone else passes nearby.

Clearly, the time constant of the detector network R4 C4 must be longenough for the hold not to be likely to disappear between individualwords of the message, and short enough for terminating the hold at theend of the message, e.g. after 2 seconds of silence, so that the systemis restored to standby for someone else to pass by. 7

Of course, recording using a continuous tape can be replaced by anyother kind of sound recording, such as one using a disk with automaticreturn of the pickup arm.

I claim:

1. A system for detecting movements in a monitored space, comprising:

an antenna and a counterpoise therefor jointly defining the space to bemonitored;

an oscillator provided with a tank circuit tuned to a predeterminedradio frequency;

a tuned circuit with a resonance frequency differing from saidpredetermined radio frequency coupled to said oscillator forenergization thereby, said tuned circuit being connected between saidantenna and said counterpoise whereby said resonance frequency iscodetermined by the effective capacitance between said antenna and saidcounterpoise, said effective capa ems g filllllbyjl ovemenLtQl fijllfiilltt in a sense increasing the difference between said resonancefrequency and said predetermined frequency;

a feedback path between said tuned circuit and said oscillator fordelivering to an input of said oscillator a control voltage establishingan operating frequency intermediate said predetermined radio frequencyand said resonance frequency to be radiated by said antenna, saidcontrol voltage varying with changes in said effective capacitance forshifting said operating frequency toward said predetermined radiofrequency in response to a movement to be detected; and

a load circuit connected to said tuned circuit, said load circuitincluding responder means for indicating a voltage change in said tunedcircuit due to a movement to be detected.

2. A system as defined in claim 1 wherein said load circuit includes adifferentiation network of long time constant in series with saidresponder means.

3. A system as defined in claim 2, further comprising diode means insaid differentiation network for bypassing voltage changes in said tunedcircuit of a polarity opposite that due to a movement to be detected.

4. A system as defined in claim 3 wherein said differentiation networkhas a resistive branch and a capacitive branch, said diode means beingconnected across said resistive branch.

5. A system as defined in claim 2 wherein said responder means includesa normally de-energized relay and trigger means for energizing saidrelay in response to a significant voltage reduction in said tunedcircuit due to a movement to be detected, said relay and said oscillatorbeing provided with a common direct-current supply, further comprisingresistance means in series with said common supply for generating avoltage drop 9 upon energization of said relay to intensify saidsignificant voltage reduction.

6. A system as defined in claim wherein said differentiation networkincludes a capacitor and a resistor, said load circuit furthercomprising a rectifying connection and an integrating network insertedbetween said tuned circuit and said capacitor for charging the latterupon occurrence of said significant voltage reduction, said resistorenabling delayed discharging of said capacitor upon restoration of saideffective capacitance to normal whereby said relay remains energizedbeyond said restoration.

7. A system as defined in claim 6, further comprisin a source oftemporary holding voltage for said trigger means and activating meanscontrolled by said relay for making said source operative over a limitedperiod.

8. A system as defined in claim 7 wherein said source comprises arecording medium carrying a message to be announced and circuitry forderiving said holding voltage from message signals in the output of saidrecording means.

9. A system as defined in claim 1 wherein said feedback path isconnected to said tuned circuit at a point whose alternating voltagedecreases upon variation of said effective capacitance by a movement tobe detected, thereby diminishing said control voltage in response tosuch movement.

10. A system as defined in claim 9 wherein said oscillator comprises atransistor with a base lead connected to said tank circuit for receivinga regenerative feedback voltage therefrom, said feedback path includinga reactance connected to said base lead for superimposing said controlvoltage upon said regenerative feedback voltage.

11. A system as defined in claim 7 wherein said relay is provided withcontact means for delivering to said trigger means a holding voltagemaintaining said relay energized.

1. A system for detecting movements in a monitored space, comprising: anantenna and a counterpoise therefor jointly defining the space to bemonitored; an oscillator provided with a tank circuit tuned to apredetermined radio frequency; a tuned circuit with a resonancefrequency differing from said predetermined radio frequency coupled tosaid oscillator for energization thereby, said tuned circuit beingconnected between said antenna and said counterpoise whereby saidresonance frequency is codetermined by the effective capacitance betweensaid antenna and said counterpoise, said effective capacitance beingvariable by a movement to be detected in a sense increasing thedifference between said resonance frequency and said predeterminedfrequency; a feedback path between said tuned circuit and saidoscillator for delivering to an input of said oscillator a controlvoltage establishing an operating frequency intermediate saidpredetermined radio frequency and said resonance frequency to beradiated by said antenna, said control voltage varying with changes insaid effective capacitance for shifting said operating frequency towardsaid predetermined radio frequency in response to a movement to bedetected; and a load circuit connected to said tuned circuit, said loadcircuit including responder means for indicating a voltage change insaid tuned circuit due to a movement to be detected.
 2. A system asdefined in claim 1 wherein said load circuit includes a differentiationnetwork of long time constant in series with said responder means.
 3. Asystem as defined in claim 2, further comprising diode means in saiddifferentiation network for bypassing voltage changes in said tunedcircuit of a polarity opposite that due to a movement to be detected. 4.A system as defined in claim 3 wherein said differentiation network hasa resistive branch and a capacitive branch, said diode means beingconnected across said resistive branch.
 5. A system as defined in claim2 wherein said responder means includes a normally de-energized relayand trigger means for energizing said relay in response to a significantvoltage reduction in said tuned circuit due to a movement to bedetected, said relay and said oscillator being provided with a commondirect-current supply, further comprising resistance means in serieswith said common supply for generating a voltage drop upon energizationof said relay to intensify said significant voltage reduction.
 6. Asystem as defined in claim 5 wherein said differentiation networkincludes a capacitor and a resistor, said load circuit furthercomprising a rectifying connection and an integrating network insertedbetween said tuned circuit and said capacitor for charging the latterupon occurrence of said significant voltage reduction, said resistorenabling delayed discharging of said capacitor upon restoration of saideffective capacitance to normal whereby said relay remains energizedbeyond said restoration.
 7. A system as defined in claim 6, furthercomprising a source of temporary holding voltage for said trigger meansand activating means controlled by said relay for making said sourceoperative over a limited period.
 8. A system as defined in claim 7wherein said source comprises a recording medium carrying a message tobe announced and circuitry for deriving said holding voltage frommessage signals in the output of said recording means.
 9. A system asdefined in claim 1 wherein said feedback path is connected to said tunedcircuit at a point whose alternating voltage decreases upon variation ofsaid effective capacitance by a movement to be detected, therebydiminishing said control voltage in response to such movement.
 10. Asystem as defined in claim 9 wherein said oscillator comprises atransistor with a base lead connected to said tank circuit for receivinga reGenerative feedback voltage therefrom, said feedback path includinga reactance connected to said base lead for superimposing said controlvoltage upon said regenerative feedback voltage.
 11. A system as definedin claim 7 wherein said relay is provided with contact means fordelivering to said trigger means a holding voltage maintaining saidrelay energized.